Life on the tidal flat

At first glance, the tidal flat looks barren and desolate, with its vast, uncovered sandy shoals. But appearances are deceptive. Under the surface, there is a veritable myriad of life, surpassed by very few other animal communities in the world.

Compared with an average seabed, the tidal flat comprises ten-times more animals by weight. With its total area of 4,700 km2, the tidal flat forms a gigantic food basket. The conditions for the rich animal life are primarily caused by the tidal waters, which bring nutrient-rich waters into the shallow Wadden Sea twice a day. The foundation for life is high occurrences of microscopic algae, especially diatoms, that comprise the first link in the food chain. The diatoms appear floating in the water, on the surface of the tidal flat and buried below the mud. The diatoms give the tidal flat its brown colour.

Varied seabed creates life

Most life in the Wadden Sea is buried below the tidal silt. However, there has to be the right mixture of sand and clay in order to create the silt. The largest quantity of animals is found in places where silt constitutes one-fifth of the seabed. If the seabed consists of pure sand or pure silt, there are only few animals. Most of the animals in the tidal flat are very small, but they often appear in huge quantities. One cubic centimetre of the tidal flat may contain more than 100 roundworms. Above the surface of the tidal flat, we find the laver spire shell snail (hydrobia ulvae) which is only a few millimetres in length and is therefore often overlooked. However, there are huge populations of these small snails. Almost 120,000 snails can live on just one square metre.

The builders of the tidal flat

One of the most numerous species of the tidal flat is the small mud shrimp, which is found in populations of almost 100,000 individuals per square metre. The mud shrimp lives in burrows in the seabed and eats decomposed plant material and bacteria, which it filters from the water or collects from the seabed. The digging

behaviour of the mud shrimp is important for the chemical decomposition of the tidal flat and the stability of the seabed. The lugworm is also important for the tidal flat. Evidence of its activity, the characteristic swirls of sand, can be found everywhere. The lugworm lives in the tidal mud in a U-shaped tunnel. It eats sand and exploits the microscopic algae found on the surface of the sand particles. The lugworm’s burrows lead water into the sand, which makes it possible for many other organisms to have their habitat deep down in the seabed.

A kindergarten for fish

The Wadden Sea is a growing site for several species of fish. The plaice spawns during winter in the open waters south-west of the Wadden Sea. From there, the fry flow with the current into the Wadden Sea, where they live for 2-3 years and grow bigger by gorging on the infinite supply of prey, before they move on to the North Sea. Sole and herring also use the Wadden Sea as a kindergarten. Lugworms are an important source of food for the fish. The lugworms deposit faecal mounds by sticking their tail out of their protective burrows, and when they do this, they are particularly exposed. The fish take advantage, but if a fish gets a good bite on the lugworm’s tail, the tail will fall off. The lugworm is ‘off the hook’, but has lost its tail. The lugworm can survive losing its tail many times.

A pantry for birds

The rich animal life on the tidal flat is a sumptuous feast for water birds. The Wadden Sea is an important resting area for many species of birds migrating from their northern breeding grounds to southern wintering areas. 12 million birds pass through the Wadden Sea every year.

The birds make a short stop to feed before they continue their journey, or they decide to stay in the area for the winter.

The course of the year on the tidal flat

Life on the tidal flat follows the changing seasons. In the summer, there is a rich animal life on the tidal flat. The temperature is favourable, and food is abundant. When winter comes, many species leave the Wadden Sea to migrate to warmer and deeper waters, but they return in the spring when the temperature rises again. Animals that cannot leave the area have a tough time during severe winters. Cockles are not very tolerant of cold weather, and in cold winters, much of the population will die on the tidal flat. However, spawn from deeper water will soon repopulate the tidal flat.

Experience the tidal flat

There are good opportunities to observe life on the tidal flat at close hand, with experiences for all tastes and preferences. If you are up for it, you can easily explore the area on your own. It is a good idea to bring a fork to dig up mussels. Exploring the hidden life of the tidal flat can be very exciting, but since it is not safe to go far out on the tidal flat, it is a good idea to be well-prepared to avoid unpleasant surprises. The high tide can quickly return to the Wadden Sea, and it is easy to lose track of time when you are walking around on the tidal flat. Therefore, it is recommended that you plan your trip according to the tide tables and decide in advance when it is time to go back. Sea fog can also roll in surprisingly fast and completely cover the area, making it very hard to find your way around. A compass or a GPS receiver are therefore essential equipment in order not to get lost.